It is common practice in the office printer and copier industry to make a lower priced copier or printer with a toner dispensing portion that is nominally a disposable cartridge. This cartridge commonly includes a toner bin, as well as a combination of a doctor blade and a gear-driven magnetic roller that is used to meter the toner onto a charged photoconductive surface.
The cost of replacing nominally disposable cartridges can be a significant fraction of the total cost of ownership. These cartridges commonly retail for 10-20% of the price of a new printer or copier. This high cost has given rise to an entire sub-industry of toner cartridge recyclers who recharge (and sometimes rebuild) the cartridges for a fraction of their new cost.
Although a cartridge can be refilled with toner many times, the precise mechanical and electrical components of the toner cartridge wear out and soon produce a visible degradation of copy quality. Since the cartridge is designed with the expectation of a short service life, serious wear problems can be encountered after only a few rechargings. Diagnosing just what wear mechanism causes which sort of copy degradation is a challenge for the re-builder, who must seek for his answers in an area unanticipated by the original designers. An example of such a diagnostic and corrective procedure is presented in a related patent application by the inventor. In his U.S. application, Ser. No. 07/823,290, the disclosure of which is herein incorporated by reference, the inventor described method and apparatus for maintaining copy quality by providing a substitute for a worn electrical contact to the toner roller.
One of the quality issues that has not been successfully addressed by the toner cartridge re-building industry is that of the "right side problem" in which a dark streak or gray background appears towards the right side of the imprinted surface of a piece of paper that has been processed through an affected electrostatic copier or printer. Re-builders have observed that when a printer or copier exhibits this effect, there is usually a build-up of excess toner on the corresponding "right side" of the corona wire (located adjacent the toner roller). Toner build-up on the corona wire can partially electrostatically shield the photoconductive drum, which prevents the corresponding "right side" of the photoconductor surface from being fully charged thus causing the observed dark streak on the right side of the paper.
Some re-builders have hypothesized that excess toner is blown onto this portion of the corona wire by a cooling fan (an exhaust vent for the air stream is normally adjacent the "right side" end of the corona wire) and have suggested placing a permanent magnet adjacent the corona wire to preferentially capture the toner (which is ferromagnetic) that would otherwise get on the wire.
The inventor has found no one in the industry who has suggested solving the "right side" problem by controlling the gap between the doctor blade and the toner roller and thereby preventing excess toner from getting out of the cartridge in the first place. This is not surprising, since (as will become apparent in the following discussion) the gap may open excessively only during printer operation, but remain within specification when the unit is at rest.